With the increase in the amount of transmission information, optical interconnect lines in addition to electrical interconnect lines have been used in recent electronic devices and the like. An opto-electric hybrid board in which an electric circuit part with an optical element mounted therein and an optical waveguide part are integrally provided has been used for various purposes.
The opto-electric hybrid board has a structure such that the optical waveguide part including an optical waveguide is provided along the back surface of a substrate and the electric circuit part having the electrical interconnect lines as a base is provided on the front surface thereof, and such that the optical element is mounted in a predetermined position in the electric circuit part. When the optical element is a light-receiving element, light transmitted in the optical waveguide is reflected from a mirror part provided at a downstream end of the optical waveguide to change its direction, and is then optically coupled to a light-receiving part of the optical element mounted on the front surface of the substrate. When the optical element is a light-emitting element, light emitted from the light-emitting element toward the back surface of the substrate is reflected from a mirror part provided at an upstream end of the optical waveguide to change its direction, and then enters the optical waveguide part.
To achieve high light propagation efficiency in the optical coupling between the optical element and the optical waveguide, it is important that the optical axis in the optical waveguide part and the optical axis of the light-receiving or light-emitting part of the optical element are precisely aligned with each other. To this end, it is necessary that the position of the mirror part that reflects light for the optical coupling and the position of the optical element mounted on an electric circuit are determined as precisely as possible in the production of the opto-electric hybrid board.
A variety of alignment methods have been proposed to determine the position of the mirror part and the position of the optical element with as high accuracy as possible. For example, the present assignee/applicant has proposed a structure of an opto-electric hybrid board and a method of manufacturing the same in which alignment marks are provided on both the front and back surfaces of a substrate in the opto-electric hybrid board so that the accuracy of the mounting position of the optical element is increased (see PTL 1, for example).
Specifically, as shown in FIG. 7, the aforementioned opto-electric hybrid board includes an electric circuit portion 1 and an optical waveguide portion 2. The electric circuit portion 1 includes a metal substrate 10, an insulation layer (not shown), an electric circuit 11, and a second alignment mark 15. The electric circuit 11 includes an optical element mounting pad 11a, and an optical element 3 is mounted on the optical element mounting pad 11a. The optical waveguide portion 2 is provided on the back surface of the substrate 10, with an adhesive layer 5 therebetween. The optical waveguide portion 2 includes a transparent under cladding layer 21, a core 22 for an optical path, a first alignment mark 24 positioned relative to an end portion 22a (serving as a mirror part for light reflection) of the core 22, and an over cladding layer 23 covering the core 22 and the first alignment mark 24. An identifying mark for the second alignment mark 15 is positioned with respect to the first alignment mark 24 on the back surface of the substrate. A through hole 12 for optical coupling is provided in the substrate 10. The optical path of light L is indicated by arrows in the figure. The reference numeral 14 designates a through hole for visual recognition of the first alignment mark 24 therethrough from the front surface side of the substrate.
According to this configuration, the first alignment mark 24 is positioned relative to the end portion 22a of the core 22, and the second alignment mark 15 is positioned with respect to the first alignment mark 24 relative to the end portion 22a of the core 22. Thus, the mounting position of the optical element 3 is practically positioned relative to the end portion 22a of the core 22. This provides increased positioning accuracy, as compared with the individual positioning of the first alignment mark 24 and the second alignment mark 15.